Over the past two to three decades, two significant technological advances have helped to enhance the mobility of transfemoral amputees. In the 1980s, the introduction of composites such as carbon fiber allowed the creation of energy-storing ankle-foot complexes that can return some of the energy stored in the stance phase of gait back to the limb for the swing phase. In the 1990s, the integration of microprocessor control with modulated damping elements in prosthetic knee joints enhanced the capability of prosthetic knees to accommodate variation in gait speed and locomotion activity. Despite these advances, the capabilities of these joints remain inferior relative to a healthy joint in a sound leg of an amputee, particularly during stair ascent and descent.
The inability to provide biomechanically healthy stair ascent is largely due to the fact that the emulation of healthy stair ascent requires significant net positive power at the knee and ankle joints. An energetically passive prosthesis is fundamentally unable to provide such net power at either joint. However, it should be noted that the inability of existing prostheses to provide biomechanically healthy stair descent is not due to a lack of power generation capability, per se, but rather due to the inability of existing prostheses to appropriately configure the ankle joint prior to foot strike.
Stair descent is characterized by forefoot strike rather than heel strike, which enables the ankle joint to dissipate substantial power during the loading phase of gait. A typical passive (compliant) ankle/foot prosthesis is unable to provide the appropriate ankle posture during terminal swing phase to set up forefoot strike, and is similarly unable to absorb energy (without later releasing that energy) during the loading phase of stair descent.